In a backward progress of a straight stitch, fine differences are caused by chance due to, for example, conditions of a stitching thread. These differences make a perfect stitch which is the same stitch as the ordinary stitch seen in FIG. 1(A), a hitch stitch as in FIG. 1(B) or an undesirable mixed stitch of both as shown in FIG. 1(C).
Formation of these stiches will be explained in reference to FIGS. 2 and 3 of the attached drawings. As shown in FIG. 2 (A), just before a needle 1 gets out above a surface of a needle plate (not shown), a hook 3 of a horizontal bobbin carrier 2 catches an upper thread 4, which rotates counterclockwise as indicated by the arrow. At this time, a stitch forming side 4a of the upper thread 4 passes along the left side of the needle 1 and penetrates the fabric 5 and passes through a needle eye 6 from its rear side toward the front side. When a thread supply side 4b thereof is at the right side of the stitch forming side 4a, the upper thread 4 simply crosses with a lower thread 7 through the conditions shown in FIGS. 2(B) and (C) in accompanyment with rotation of the sewing machine, following the arrow marks, and forms a perfect stitch as shown in FIG. 2(D). While the stitch shown in FIG. 1(A) is successively formed from a lower part to an upper part thereof, the needle 1 drops at the right side of the stitch formed at the front side, so that the perfect stitch is produced.
On the other hand, when the stitch forming side 4a passes along the right side of the needle 1, the thread supply side 4b is at the left side of the stitch forming side 4a, and the upper thread 4 crosses with the lower thread through the conditions shown in FIGS. 3(B) and (C) in accompanyment with rotation of the sewing machine, and further it also crosses with itself, and forms a hitch stitch as shown in FIG. 3(D). When the stitch is formed as shown in FIG. 1(B), the needle 1 drops at the left side of the stitch formed at the front side, so that the hitch stitch is produced.
However, in the backward progress of the straight stitch, it is rare that the stitch is united into any one of the perfect stitch or the hitch stitch, but rather as seen in FIG. 1(C), those are very often mixed alternately or irregularly.
With respect to a cause of such mixture, under the condition of FIG. 2(D), after the perfect stitch is formed and when the fabric 5 is moved toward the operator, the lower thread 7 pushes the thread supply side which is about to form a stitch, to the right side at a crossing point 8, and therefore the thread supply side 4b gets out at a position toward the right side from the center line, so that the thread supply side 4b is directed to be easily at the right side of the needle 1, and a subsequent stitch is very often made a hitch stitch.
Under the condition shown in FIG. 3(D), since the upper thread 4 pushes the thread supply side 4b to the left side by crossing with itself, the needle drops at the right side of the thread, and a subsequent stitch is very often made a perfect stitch.
Those stitches are not caused by simple causes, and if they are formed alternately or irregularly, the finished stitches do not look appealing. As countermeasures therefor, special processes have been considered to the bobbin carrier, the needle plate or the fabric presser, but unfortunately satisfactory results have not yet been obtained.